The present invention relates to a composite material with low thermal expansivity and high thermal conductivity, to a process for production thereof, and to applications thereof such as semiconductor devices.
Power electronics include a technique which deals with power electronic devices to convert and control electric power and energy, power electronic devices used in on-off mode, and a power conversion system as its application technique.
Conversion of electric power calls for a variety of power semiconductors with switching capability. These semiconductors in practical use include rectifier diodes (with a pn junction for current flow in only one direction), and thyristors, bipolar transistors, and MOSFETs (with a combination of pn junctions). Recently developed ones include insulated gate bipolar transistors (IGBT) and gate turn-off thyristors (GTO) which perform switching in response to gate signals.
These power semiconductors evolve heat when energized. They tend to evolve more heat with their increasing capacity and speed. To protect them from deterioration and life-shortening due to heat evolution, they should be provided with a radiator which prevents temperature rise in themselves and in their vicinity. A common material used for radiators is copper, which is inexpensive and has high thermal conductivity (393 W/m). Unfortunately, copper is not suitable for the radiator of power semiconductor devices because it has a high thermal expansivity of 17xc3x9710xe2x88x926/xc2x0 C. and hence it is not soldered well with silicon whose thermal expansivity is 4.2xc3x9710xe2x88x926/xc2x0 C. One way to address this problem is to make the radiator from molybdenum or tungsten which has thermal expansivity close to that of silicon or to interpose it between the radiator and the semiconductor element.
Power semiconductor elements are contrasted with electronic semiconductor elements. The latter are exemplified by integrated circuits (IC) consisting of electronic circuits integrally formed on a single semiconductor chip. They are classified into memory, logic, microprocessors, etc. according to their functions. A problem involved with recent electronic semiconductor elements is heat evolution, which increases as the degree of integration increases and the speed of operation increases. To make things worse, electronic semiconductor elements are contained individually in hermetic packages for isolation from the atmosphere to prevent failure and deterioration. Widespread packages are ceramic packages (in which each semiconductor element is fixed to ceramics through die bonding) and plastics packages (which are sealed with plastics). A new development to meet requirements for high reliability and high speed operation is the multi-chip module (MCM) equipped with a plurality of semiconductor elements on a single substrate.
A plastics package is constructed such that the semiconductor element therein has its terminals connected to the lead frame through bonding wires and the entire assembly is sealed with plastics. Recent improvements made to cope with increasing heat evolution are packages in which the lead frame functions to dissipate heat or which are provided with a radiator for heat dissipation. The lead frame or radiator for heat dissipation is usually made of copper with high thermal conductivity. Unfortunately, malfunction is anticipated because of difference in thermal expansivity between copper and silicon.
By contrast, ceramics packages are constructed such that a semiconductor element is placed on a ceramic substrate having wiring printed thereon and the entire assembly is sealed with a metal or ceramics cap. The ceramic substrate is backed with Cuxe2x80x94Mo or Cuxe2x80x94W composite material or kovar alloy, which functions as a radiator. Ceramic materials with low thermal expansivity, high thermal conductivity, and good workability are required at low production cost.
An MCM consists of a metal or ceramic substrate having thin film wiring formed thereon, a plurality of semiconductor elements (in the form of bare chip) mounted thereon, a ceramic package containing them, and a sealing lid. The package is provided with a radiator or fin if it needs heat dissipation. The metal substrate is made of copper or aluminum. The have the advantage of high thermal conductivity but also have the disadvantage of high thermal expansivity, which leads to poor matching with the semiconductor element. Therefore, the substrate of MCMs for high reliability is made of silicon or aluminum nitride (AlN). The radiator, which is bonded to the ceramic package, should be made of a material which has high thermal conductivity and also has low thermal expansivity for good matching with the package material.
As mentioned above, all semiconductor devices evolve heat during operation and are subject to malfunction if heat is accumulated. Therefore, they need a radiator with good thermal conductivity for heat dissipation. The radiator, which is bonded to the semiconductor element directly or indirectly through an insulating layer, calls for not only high thermal conductivity but also low thermal expansivity for good matching with the semiconductor element.
Prevailing semiconductor elements are based on Si or GaAS, which have a coefficient of thermal expansion of 2.6xc3x9710xe2x88x926 to 3.6xc3x9710xe2x88x926/xc2x0 C. and 5.7xc3x9710xe2x88x926 to 6.9xc3x9710xe2x88x926/xc2x0 C., respectively. Among known materials comparable to them in thermal expansivity are AlN, SiC, Mo, W, and Cuxe2x80x94W. When used alone for radiators, they do not permit their heat transfer coefficient and thermal conductivity to be controlled as desired. They are poor in workability and high in production cost. A Cuxe2x80x94Mo sintered alloy is proposed in Japanese Patent Laid-open No. Hei 8-78578. A Cuxe2x80x94Wxe2x80x94Ni sintered alloy is proposed in Japanese Patent Laid-open No. Hei 9-181220. A Cuxe2x80x94SiC sintered alloy is proposed in Japanese Patent Laid-open No. Hei 9-209058. An Alxe2x80x94SiC composite material is proposed in Japanese Patent Laid-open No. Hei 9-15773. These conventional composite materials permit their heat transfer coefficient and thermal conductivity to be controlled over a broad range if the ratio of their constituents is changed. However, they are poor in plastic workability and hence they present difficulties in making into thin plate and need many manufacturing steps.
It is an object of the present invention to provide a composite material having low thermal expansivity, high thermal conductivity, and good plastic workability, a semiconductor device made with said composite material, a radiator for said semiconductor device, an electrostatic attractor, and a dielectric plate for said electrostatic attractor.
The first aspect of the present invention resides in a composite material composed of metal and inorganic particles having a smaller coefficient of thermal expansion than said metal, characterized in that said inorganic particles disperse in such a way that 95% or more of them (in terms of their area in cross-section) form aggregates of complex configuration joining together.
The second aspect of the present invention resides in a composite material composed of metal and inorganic particles having a smaller coefficient of thermal expansion than said metal, characterized in that said inorganic particles are individually present such that they count 100 or less in a sectional area of 100 xcexcm square, with the remainder dispersing in the form of aggregates of complex configuration joining together.
The third aspect of the present invention resides in a composite material composed of metal and inorganic particles having a smaller coefficient of thermal expansion than said metal, characterized in that said inorganic particles are have a Vickers hardness of 300 or less. The fourth aspect of the present invention resides in a composite material composed of metal and inorganic particles having a smaller coefficient of thermal expansion than said metal, said composite material having a coefficient of thermal expansion which increases by 0.025-0.035 ppm/xc2x0 C. on average per W/mxc2x7K at 20xc2x0 C. in the range of 20-105xc2x0 C.
The fifth aspect of the present invention resides in a composite material composed of metal and inorganic particles having a smaller coefficient of thermal expansion than said metal, characterized in that said inorganic particles disperse in the form of aggregates joining together, said aggregates elongating in the direction of plastic working.
The sixth aspect of the present invention resides in a composite material composed of copper and copper oxide particles, characterized in that said copper oxide particles disperse in such a way that 95% or more of them (in terms of their area in cross-section) form aggregates of complex configuration joining together.
The seventh aspect of the present invention resides in a radiator plate for a semiconductor device which is made of said composite material.
The eighth aspect of the present invention resides in a radiator plate for a semiconductor device which has a nickel plating layer thereon.
The ninth aspect of the present invention resides in a semiconductor device which comprises a plurality of insulating substrates and a plurality of semiconductor elements mounted on each of said insulating substrates, each of said insulating substrates having said radiator plate directly joined to said insulating substrate through a conductive layer formed on the upper and lower surfaces of said insulating substrate.
The tenth aspect of the present invention resides in a semiconductor device which comprises an insulating substrate with a radiator plate and a semiconductor element mounted on said insulating substrate, wherein said radiator plate is the one defined in the seventh or eighth aspect.
The eleventh aspect of the present invention resides in a semiconductor device which comprises a semiconductor element mounted on a radiator plate, a lead frame joined to said radiator plate, and metal wiring to electrically connect said lead frame with said semiconductor element, said semiconductor element being sealed with plastics, wherein said radiator-plate is the one defined in the seventh or eighth aspect.
The twelfth aspect of the present invention resides in a semiconductor device which comprises a semiconductor element mounted on a radiator plate, a lead frame joined to said radiator plate, and metal wiring to electrically connect said lead frame with said semiconductor element, said semiconductor element being sealed with plastics and said radiator plate being open at the side opposite to the side to which said semiconductor element is joined, wherein said radiator plate is the one defined in the seventh or eighth aspect.
The thirteenth aspect of the present invention resides in a semiconductor device which comprises a semiconductor element mounted on a radiator plate, pins for connection with external wiring, a ceramics multilayer wiring substrate having at its center an open space to hold said semiconductor element, and metal wiring to electrically connect said semiconductor element with the terminals of the substrate, said radiator plate and said substrate being joined to each other such that said semiconductor element is installed in said space and said substrate being joined to a lid such that said semiconductor element is isolated from the atmosphere, wherein said radiator plate is the one defined in the seventh or eighth aspect.
The fourteenth aspect of the present invention resides in a semiconductor device which comprises a semiconductor element mounted on a radiator plate, terminals for connection with external wiring, a ceramics multilayer wiring substrate having at its center a recess to hold said semiconductor element, and metal wiring to electrically connect said semiconductor element with the terminals of the substrate, said radiator plate and the recess of said substrate being joined to each other such that said semiconductor element is installed in said recess and said substrate being joined to a lid such that said semiconductor element is isolated from the atmosphere, wherein said radiator plate is the one defined in the seventh or eighth aspect.
The fifteenth aspect of the present invention resides in a semiconductor device which comprises a radiator plate, a semiconductor element joined onto said radiator plate with a thermally conductive resin, a lead frame joined to a ceramics insulating substrate, and a TAB to electrically connect said semiconductor element with the lead frame, said radiator plate and said substrate being joined to each other such that said semiconductor element is isolated from the atmosphere, and said semiconductor element and said insulating substrate being separated by a thermally conductive elastic resin interposed between them, wherein said radiator plate is the one defined in the seventh or eighth aspect.
The sixteenth aspect of the present invention resides in a semiconductor device which comprises a first radiator plate, a semiconductor element joined to said radiator plate with metal, a second radiator plate joined to a grounding plate, said first radiator plate being mounted on the grounding plate of the radiator plate, and a TAB electrically connected to the terminals of said semiconductor element, said semiconductor element being sealed with plastics, wherein said radiator plate is the one defined in the seventh or eighth aspect.
The seventeenth aspect of the present invention resides in a dielectric plate for electrostatic attractors which is made of the composite material defined in any of the first to sixth aspects mentioned above.
The eighteenth aspect of the present invention resides in an electrostatic attractor which comprises an electrode layer and a dielectric plate bonded to said electrode layer, said dielectric producing an electrostatic attractive force upon application of a voltage to said electrode layer such that an object is fixed onto the surface of said dielectric plate, wherein said dielectric plate is the one defined in the seventeenth aspect.
The composite material according to the present invention is composed of metal and inorganic particles. The metal includes Au, Ag, Cu, and Al, among which Cu is the most desirable because of its high melting point and high strength. The inorganic particles should preferably be those which are comparatively soft and stable after sintering and have an average coefficient of thermal expansion equal to or smaller than 5.0xc3x9710xe2x88x926/xc2x0 C., preferably equal to or smaller than 3.5xc3x9710xe2x88x926/xc2x0 C., in the range of 20-150xc2x0 C., and also have a Vickers hardness of 300 or less. (They are different from conventional ones., such as SiC and Al2O3, which greatly differ in hardness from the matrix metal.) Such soft inorganic particles provide good plastic workability (either hot or cold) after sintering. Rollability makes it possible to produce a comparatively thin plate in a short processing time. The resulting composite material has a high strength because of the inorganic particles dispersed therein. Conceivable examples of the inorganic particles include copper oxide, tin oxide, lead oxide, and nickel oxide. Of these examples, copper oxide is preferably because of the smallest coefficient of thermal expansion.
The composite material of the present invention should preferably be reinforced with hard, fine ceramics particles, such as SiC and Al2O3, having a Vickers hardness of 1000 or more and an average particle diameter of 3 xcexcm or less, in an amount of 5 vol % or less.
The radiator plate and dielectric plate according to the present invention may be obtained in its final shape by sintering, optional rolling, and plastic working (such as pressing).
The composite material according to the present invention should preferably be a copper (Cu) alloy containing cuprous oxide (Cu2O) in an amount of 20-80 vol %, with the Cu phase and the Cu2O phase forming the dispersing structure. The composite material should preferably have a coefficient of thermal expansion of 5xc3x9710xe2x88x926 to 14xc3x9710xe2x88x926/xc2x0 C. and a thermal conductivity of 30-325 W/mxc2x7K in the range of room temperature to 300xc2x0 C.
The copper-cuprous oxide composite material should preferably contain cuprous oxide (Cu2O) in an amount of 20-80 vol %, with the remainder being copper (Cu). The Cu2O phase and the Cu phase should have an oriented structure. The composite material should preferably have a coefficient of thermal expansion of 5xc3x9710xe2x88x926 to 14xc3x9710xe2x88x926/xc2x0 C. and a thermal conductivity of 30-325 W/mxc2x7K in the range of room temperature to 300xc2x0 C. The thermal conductivity in the direction of orientation should be greater than twice that in the direction perpendicular to the direction of orientation.
The composite material according to the present invention is produced by steps of mixing copper powder and cuprous oxide powder, pressing the mixed powder, sintering the pressed form at 800-1050xc2x0 C., and performing cold or hot plastic working. (Copper powder is an example of said metal and cuprous oxide powder is an example of said inorganic particles.)
The copper composite material according to the present invention is produced from a mixed powder of cupric oxide (CuO) and copper (Cu) containing inevitable impurities. The amount of cupric oxide is 10.8-48.8 vol %. The production process consists of steps of press-forming the mixed powder, sintering the pressed form at 800-1050xc2x0 C., thereby solidifying the pressed form and forming Cu2O by reaction between CuO and Cu, hot or cold pressing (for plastic working), and annealing.
The copper composite material of the present invention is composed of Cu and Cu2O, the former having a high coefficient of thermal expansion of 17.6xc3x9710xe2x88x926/xc2x0 C. and a thermal conductivity as high as 391 W/mxc2x7K, the latter having a low coefficient of thermal expansion of 2.7xc3x9710xe2x88x926/xc2x0 C. and a thermal conductivity of 12 W/mxc2x7K. It is formed into a radiator plate for semiconductor devices by sintering. The sintered body is composed of Cu and Cu2O in an amount of 20-80 vol %. It has a coefficient of thermal expansion of 5xc3x9710xe2x88x926 to 14xc3x9710xe2x88x926/xc2x0 C. and a thermal conductivity of 30-325 W/mxc2x7K in the range of room temperature to 300xc2x0 C. With Cu2O in an amount of 20% or more, the composite material has a high coefficient of thermal conductivity required of the radiator plate. With Cu2O in an amount of 80% or less, the composite material has sufficient thermal conductivity and structural strength.
The composite material according to the present invention is obtained basically by powder metallurgy. The copper composite material is obtained from Cu powder and Cu2O powder or CuO powder. These powders (as raw materials) are mixed in a prescribed ratio, the mixed powder is cold-pressed in a mold, and the resulting preform is sintered. If necessary, the sintered body undergoes hot or cold plastic working.
The mixing of raw material powders is accomplished by using a V-mixer, pot mill, or mechanical alloying. The particle size of the raw material powders affect the press molding performance and the dispersibility of Cu2O after sintering. Therefore, the Cu powder should have a particle diameter of 100 xcexcm or less, and the Cu2O and CuO powder should have a particle diameter of 10 xcexcm or less, preferably 1-2 xcexcm.
The mixed powder undergoes cold pressing in a mold under a pressure of 400-1000 kg/cm2. The pressure should preferably be increases in proportion to the Cu2O content.
The preform of the mixed powder is sintered is an argon atmosphere under normal pressure or sintered by HIP or hot pressing under pressure. Sintering should be carried out at 800-1050xc2x0 C. for about 3 hours. The sintering temperature should be increased in proportion to the Cu2O content. The sintering temperature varies depending on the kind of the matrix metal. In the case of copper, the sintered body will have a low density if the sintering temperature is 800xc2x0 C. or less. In addition, sintering at a temperature of 1050xc2x0 C. or more brings about a eutectic reaction between Cu and Cu2O, which would result in partial melting. Therefore, the adequate sintering temperature ranges from 900xc2x0 C. to 1000xc2x0 C.
The copper composite material according to the present invention is composed of Cu and Cu2O, which have a low hardness. Therefore, it is capable of cold or hot working, such as rolling and forging, which is carried out after sintering, if necessary. Working leads to anisotropic thermal conductivity, which contributes to strength or some applications which need heat conduction in a specific direction.
According to the present invention, the raw material powder may be CuO. This CuO powder is mixed with Cu powder and the mixed powder is press-formed. The resulting preform is sintered so that Cu is oxidized. Thus there is obtained a sintered body which is composed of a matrix of Cu and a dispersed phase of Cu2O. CuO coexisting with Cu transforms into Cu2O (which is thermal stable) at high temperatures according to equation (1) below.
2Cu+CuOxe2x86x92Cu+Cu2Oxe2x80x83xe2x80x83(1)
A certain length of time is required before an equilibrium is reached in the reaction represented by the equation (1). About 3 hours will be sufficient if the sintering temperature is 900xc2x0 C.
Cu2O particles in the sintered body should be as fine as possible because their particle diameter affects the density, strength, and plastic workability of the composite material. The particle diameter is greatly affected by the mixing method. The larger the mixing energy, the less the coagulation of powder particles. Thus fine Cu2O particles are obtained after sintering.
According to the present invention, the particle size of the Cu2O phase is established as follows depending on the mixing machine employed. 50 vol % or more of particles should have a particle diameter of 50 xcexcm or less if a V-mixer is used (with small mixing energy), a particle diameter of 50 xcexcm or less if a pot mill containing steel balls is used, and a particle diameter of 10 xcexcm or less if mechanical alloying (with the largest mixing energy) is employed, with the remainder having a particle diameter of 50-200 xcexcm. With a particle diameter of 200 xcexcm or more, the resulting composite material has high porosity and hence is poor in plastic workability. With a content of Cu2O phase in excess of 50 vol %, the resulting composite material is low in thermal conductivity and uneven in characteristic properties and hence it is inadequate for use as the radiator plate for semiconductor devices. A preferred structure is one which is composed of a Cu phase and a Cu2O phase (50 xcexcm or less) uniformly dispersed therein. Cu2O particles have an extremely irregular shape and are joined together before sintering; their particle diameter before sintering can be observed with a high magnification. The Cu2O phase should preferably 10 xcexcm or less.